The present invention relates to a press for increasing the dry solids content of a continuous web of fibrous material such as paper, and for imprinting the web with an imprinted pattern.
Pressing a wet fibrous web in a press at an elevated temperature and at a high pressure has been found to result in a substantially increased dewatering rate as well as other favorable effects. The technique, which is variously called an impulse technique, impulse pressing, or impulse drying, is described in U.S. Pat. No. 4,324,613. The maximum pressures employed in the method described in the ""613 patent lie within the range of 3-8 MPa. The ""613 patent describes a conventional roll press formed by two cylindrical press rolls. A burner supplies heat to one of the rolls. The heat is supplied to the mantle surface of the roll immediately before the press nip. The patent states that the roll can have a surface layer with low capacity of heat transmission so that the surface layer thereby can maintain a high temperature.
The roll surface temperatures are between about 150xc2x0 C. and about 350xc2x0 C. However, because the press nip is short in length, the dwell time for the paper web in this type of press nip is only a few milliseconds, and this dwell time is too short for it to derive much of the potential advantage of pressing at high temperature under simultaneously applied high pressure. Therefore, it has also been proposed to utilize impulse drying in a heated shoe press, wherein the press nip is extended to approximately 20 to 30 cm in length, so that the dwell time for the heat treatment becomes considerably longer.
Regardless of whether accomplished in a roll nip or an extended press nip, however, impulse drying in a heated press nip generally involves subjecting the web to high temperature and high pressure simultaneously in order to give very high thermal flows to the web. A heat transferring member in the form of the mantle surface of a steel or cast iron roll, which transfers heat to the fibrous web, achieves high heat transfer rates within the range of about 2-8 MW/m2, which results in very high dewatering rates. The mechanism by which these high dewatering rates are achieved is not yet entirely clear. One theory that has been submitted is that when the vapor that develops near the surface of the heat transferring member closest to the fibrous web expands, the vapor helps to force substantially all remaining water in the fibrous web into the felt that is in contact with the fibrous web. Another theory is that the high dewatering rate is achieved by a combination of the reduced viscosity of the water caused by the high temperature, which makes it easier to force the water from the web, and a rapid evaporation of water from the web that occurs when the water heated to above 100xc2x0 C. under pressure in the nip suddenly drops to atmospheric pressure when the web leaves the press nip.
U.S. Pat. No. 4,738,752 describes an extended, heated press nip in which the fibrous web encounters a hot surface defined by a rotatable press roll or by a metal belt that runs in a loop around a plurality of guide rolls. The press roll or belt is heated by a heating apparatus. The press roll can be formed to have an inner or first layer and an outer or second layer coaxially surrounding the first layer and having a coefficient of thermal conductivity that is larger than that of the first layer. The first layer may be of ceramics, while the second, external layer consists of metal and has a thickness of 0.0127-1.27 cm. The layers are in intimate contact with each other and together they constitute a unitary roll body.
When a roll of the above-described type is heated from the outside, one problem is that the external layer will become warmer than the layer or the layers located inward of the external layer. The external layer will therefore expand more than the inner layer and, because the external layer is joined with the inner layer, tensions will occur between the two layers. Even if the roll is homogeneous, differences in expansion and tensions between radially outer and radially inner portions of the roll will arise. In order to reduce the risk for damage of the roll caused by such difference in tensions, the initial heating of the roll must be done slowly. Another problem is that it is difficult to maintain the desired geometrical shape of the roll in the cross-machine direction because of the difficulties in maintaining the same temperature along the mantle surface of the roll and on the roll heads. Since the mantle surface and the heads cannot expand freely and independently of each other, large stresses occur between them and the mantle surface can become curved outwards or inwards in the cross-machine direction. Moreover, because the inner layer or portion of the roll will absorb some of the heat energy supplied for heating the outer layer or portion of the roll, the heating costs become high.
One disadvantage of using a metal belt as a heat-transferring device is that it must be arranged in a loop about at least two rolls, and hence this configuration requires considerable space. In order to clean the surface of the belt with a doctor blade, a counter roll must be arranged inside of the belt loop opposite the doctor blade. Another disadvantage is that the belt usually cannot be coated with layers in order to achieve certain properties of release and certain thermal conductivities.
In production of soft paper of relatively low basis weight, which for instance is used for making household paper, paper towels, and other hygiene products, it is generally desired to produce a bulk, i.e., a relationship between the volume and the weight of the paper, that is substantially higher than for other papers, as paper with high bulk has a desirable combination of softness and high power of absorption. In conventional production of paper, a coherent fibrous web is formed on a wire by dewatering a pulp suspension with initially very high water content. The moist web runs through a press section comprising one or more presses, each with at least one press nip, in which additional water is pressed out of the web. However, in a conventional wet press a certain re-wetting of the web inevitably occurs at the outlet of the press nip, and moreover the fibers are pressed together in a disadvantageous way such that a relatively flat and compact soft paper web with lower bulk than desired is obtained after the web has been dried in a drying section. In the manufacturing of paper, board, and cardboard, usually the drying section for drying the web has a large number of alternating drying cylinders and guide rolls around which the moist web runs. This large number of cylinders and rolls gives the paper machine a disadvantageous length.
In the manufacture of soft paper, usually a Yankee dryer has hitherto been used for the main drying of the web. The conditions at an Yankee dryer differ from those at a conventional wet press, in that the dewatering of the web is not accomplished primarily by pressing but rather is achieved chiefly by thermal drying of the web.
However, certain attempts have been made to manufacture soft paper with a bulk that is substantially increased from what is generally achievable using the conventional pressing and drying processes. For instance, U.S. Pat. No. 3,806,406 describes a procedure and a device for forming a high bulk soft paper web in which the wet web is transported on a felt through a press nip between a press roll and a heated Yankee dryer. Since compressing a fibrous web results in a lower bulk than what is desired for soft paper, the technique described in the ""406 patent aims to reduce the surfaces of the fibrous web that are exposed to pressure in the press. Accordingly, the Yankee dryer has a mantle surface with alternately raised and lowered surface portions, which constitute a relief pattern for placing against the web. Thus, only the parts of the web that are in contact with the raised surface portions are compressed in the press nip, while the parts in between are left relatively unaffected. Substantially the entire drying of the web is done thermally by supplying heat from the heated Yankee dryer, while the soft paper web is maintained in a fixed position relative to the raised and lowered surface portions. If required, a certain pre-drying or after-drying of the web can be done as well by through-air drying. The web processed in this manner obtains a certain imprinted pattern that consists of compressed and non-compressed portions corresponding to the pattern of the mantle surface. Pattern imprinting of a moist fibrous web and drying of the imprinted fibrous web without destroying the procured structure gives a high bulk and, furthermore, the structure remains even if the dried fibrous web subsequently becomes wet.
However, the Yankee dryer has some disadvantages. It has a large diameter, and hence it is very bulky. In using a Yankee dryer, it is hard to achieve the very high temperatures that are generally desired in impulse drying because the Yankee dryer is heated from its interior by steam, and even though the steam inside the drum may have a high temperature, the external mantle surface of the drum reaches only approximately 95xc2x0 C.-100xc2x0 C. Moreover, the entire Yankee dryer must be heated up, with high energy costs as a consequence. Because of the relatively low temperature achieved at the outer surface of the Yankee dryer, the web must be in contact with the Yankee dryer for a long time period, which in turn leads to the need for a large diameter, and/or to a need for either pre-drying or after-drying, and occasionally both. Additionally, the aforementioned problem of internal tensions that can arise as a result of differences in temperature between different parts of a heated roll also can occur with the heated Yankee dryer.
The present invention seeks to overcome the drawbacks noted above, by providing a press for imprinting and drying a fibrous web wherein the press nip is defined between a press device and a roll body, and the heat transferring device that heats the web in the press nip is in the form of a sleeve that encircles the roll body and is heated by at least one heating apparatus. The sleeve is in driven engagement with the roll body such that rotation of the roll body causes the sleeve to pass through the nip with the web against an external mantle surface of the sleeve, the roll body and sleeve thus collectively comprising a counter roll. The external mantle surface of the sleeve that presses against the web has imprinting elements for imparting a desired imprinted pattern in the web. The sleeve can be substantially rigid such that it is stable in form and does not substantially deform under the loads imposed on it by the press device and roll body. Alternatively, the sleeve can be a flexible and deformable sleeve. During operation when the sleeve is heated to the desired operating temperature, the sleeve preferably loosely surrounds the roll body so that the sleeve can freely expand and contract relative to the roll body. Furthermore, there preferably is no attachment, adhesion, or other fixing connection between the roll body and the sleeve that would prevent such free expansion of the sleeve. Accordingly, the invention avoids the large thermally induced stresses that could occur between outer and inner layers or portions of prior unitary roll bodies.
The sleeve can be either permeable or impermeable. The imprinting elements can have various forms, including recesses and/or grooves formed in the mantle surface partially through the thickness of the sleeve and/or through holes formed entirely through the thickness of the sleeve.
The web is preferably arranged to contact and be carried by the mantle surface of the sleeve over an angular sector of the sleeve having an angular extent of about 10xc2x0 to about 300xc2x0. The heating apparatus preferably is located ahead of the press nip by an angular spacing in the range of about 30xc2x0 to about 330xc2x0 as measured circumferentially about the sleeve.
The press call comprise either a roll press in which the press device in nipping engagement with the counter roll is a cylindrical roll, or an extended-nip press in which the press device comprises a press shoe having a concave surface and a flexible jacket that surrounds the press shoe and passes through the nip in sliding contact with the concave surface of the shoe. The endless clothing that passes through the press nip can comprise a belt having recesses therein for receiving water in liquid and vapor form from the web and transporting the water away from the web and out of the press nip, and for imparting an imprinted pattern to the web.